For hearing aid users, certain physical activities induce low-frequency vibrations that excite the hearing aid microphone in such a way that the low frequencies are amplified by the signal processing circuitry thereby causing excessive buildup of unnatural sound pressure within the residual ear-canal air volume. The hearing aid industry has adapted the term “ampclusion” for these phenomena as noted in “Ampclusion Management 101: Understanding Variables” The Hearing Review, pp. 22-32, August (2002) and “Ampclusion Management 102: A 5-step Protocol” The Hearing Review, pp. 34-43, September (2002), both authored by F. Kuk and C. Ludvigsen. In general, ampclusion can be caused by such activities as chewing or heavy footfall motion during walking or running. These activities induce structural vibrations within the user's body that are strong enough to be sensed by a MEMS accelerometer that is properly positioned within the earmold of a hearing assistance device. Another user activity that can excite such a MEMS accelerometer is simple speech, particularly the vowel sounds of [i] as in piece and [u] is as in rule and annunciated according to the International Phonetic Alphabet. Yet another activity that can be sensed by a MEMS accelerometer is automobile motion or acceleration, which is commonly perceived as excessive rumble by passengers wearing hearing aids. Automobile motion is unique from the previously-mentioned activities in that its effect, i.e., the rumble, is generally produced by acoustical energy propagating from the engine of the automobile to the microphone of the hearing aid. The output signal(s) of a MEMS accelerometer can be processed such that the device can detect automobile motion or acceleration relative to gravity. One additional user activity, not related to ampclusion, that can be detected by a MEMS accelerometer is head tilt. Finally, it should be noted that a MEMS gyrator or a MEMS microphone can be used to detect all of the above-referenced user activities instead of a MEMS accelerometer. It is understood that a MEMS acoustical microphone may be modified to function as a mechanical or vibration sensor. For example, in one embodiment the acoustical inlet of the MEMS microphone is sealed. Other techniques modifying an acoustical microphone may be employed without departing from the scope of the present subject matter. In addition to the translational acceleration estimates provided by a MEMS accelerometer, a MEMS gyrator provides three additional rotational acceleration estimates.
Thus, there is a need in the art for a detection scheme that can reliably identify user activities and trigger the signal processing algorithms and circuitry to process, filter, and equalize their signal so as to mitigate the undesired effects of ampclusion and other user activities. In all of the activities described in the previous paragraph, the MEMS device acts as a detection trigger to alert the hearing aid's signal processing algorithm to specific user activities thereby allowing the algorithm to filter and equalize its frequency response according to each activity. Such a detection scheme should be computationally efficient, consume low power, require small physical space, and be readily reproducible for cost-effective production assembly.